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The present invention relates to electrophotographic devices, such as laser printers, and in particular to the reduction of banding artifacts produced by electrophotographic printers.
Electrophotography (EP) is the basic imaging process used in paper copiers and laser printers. Conventional EP devices include an organic photoconductive (OPC) drum that rotates at a constant angular velocity. As the OPC drum rotates it is electrostatically charged, and a latent image is exposed line by line onto the OPC drum using a scanning laser, e.g., using a rotating polygon mirror. The latent image is then developed by electrostatically adhering toner particles to the OPC drum. The developed image is then transferred from the OPC drum to the output media (paper). The toner image on the paper is then fused to the paper to make the image on the paper permanent. The surface of the OPC drum is then cleaned to remove any residual toner on the surface of the OPC drum.
Typically, the EP device drives the rotating polygon mirror and the OPC drum using two brushless DC (BLDC) motors. The rotational velocity of the rotating polygon mirror can be maintained very precisely using a BLDC motor, because there is no external loading except for the mirror itself. The main drive motor for driving the OPC drum, on the other hand, has a substantial amount of external loading, because the main drive motor typically drives all the auxiliary rollers and transports the paper, particularly in a low cost EP device. The main drive motor typically drives the auxiliary rollers and paper transport through a series of gear trains. With the additional loading, as well as periodic disturbances due to imperfections in the series of gear trains, the rotational velocity of the OPC drum is difficult to control resulting in velocity perturbations.
The OPC drum velocity perturbations cause scan line spacing variation in the printed image. The scan line spacing variation is a significant contributor of artifacts in EP processes. For example, halftone banding caused by scan line spacing variation is one of the most visible and undesirable artifacts, appearing as light and dark streaks across a printed page perpendicular to the process direction. Thus, to reduce halftone banding artifacts, the OPC drum velocity variation should be reduced.
Recent improvements to EP devices to reduce halftone banding, however, have been focused on manufacturing more precise gears and better mountings to reduce or eliminate the velocity perturbations. Unfortunately, improved mechanical precision does not completely reduce banding artifacts. Moreover, mechanical components, such as gears, tend to wear with use. Consequently, as the EP device is used and the mechanical components wear, image quality due to banding artifacts will deteriorate.
Thus, what is needed is improved regulation of the OPC drum rotational velocity under various loading uncertainty and process variations, e.g., mechanical component manufacturing tolerance and wear, to improve EP process stability and reduce the appearance of banding artifacts.
An electrophotographic device uses a closed loop controller that receives a feedback signal from an encoder connected to the OPC drum to improve the rotational velocity control of the drum. The encoder provides the rotational position or angular velocity of the drum to the closed loop controller as the feedback signal. The electrophotographic device includes a rotating drum, such as an OPC drum, a motor that drives the rotating drum, the encoder that is connected to the rotating drum, and a controller that controls the motor based on the feedback signal from the encoder. In another aspect of the invention, a method of controlling the velocity of the drum to reduce banding artifacts includes providing a control signal to the motor that drives the drum, monitoring the position and/or angular velocity of the drum, and varying the control signal to the motor based on the position and/or the angular velocity of the drum.
In another aspect of the present invention, the electrophotographic device includes a closed-loop controller that controls the angular velocity of the monitor that drives the drum, wherein the closed-loop controller incorporates a model of the human visual system, such as the human contrast sensitivity function. The human contrast sensitivity function may be incorporated into the controller, e.g., in a digital signal processor or microprocessor, that helps filter out low frequency and non-periodic drum rotational velocity fluctuations that contribute to banding artifacts. In another aspect of the invention, a method of controlling the velocity of a rotating drum in the electrophotographic device to reduce banding artifacts includes providing a command signal, receiving a feedback signal of at least one of the position and angular velocity of the rotating drum, and using the command signal and the feedback signal in a primary control loop incorporating the human visual system to produce a control signal for a motor that drives the rotating drum. The command signal and feedback signal may be used to produce an error signal, which is filtered to at least partially filter out low frequency and non-periodic drum rotational velocity fluctuations to approximate the human visual system. The control signal provided to the motor is based on the filtered error signal.
In another aspect of the present invention, a closed loop controller for an electrophotographic device is designed to reduce banding artifacts by modeling an open loop transfer function for the electrophotographic device, and using loop shaping to design the closed loop controller with respect to said open loop transfer function to incorporate the human visual system model. The loop shaping techniques may be used to produce a desired frequency response in the electrophotographic device by modifying the open loop transfer function.
In another aspect of the present invention, the electrophotographic device may also include a repetitive controller in a secondary control loop to help reduce the effect of periodic drum rotational velocity fluctuations that contribute to banding artifacts. The repetitive controller may be used with a primary control loop that incorporates the human visual system or with any other control loop. The electrophotographic device, thus, includes a rotating drum, a motor that drives the rotation of the drum, an encoder coupled to the drum that monitors at least one of the position and angular velocity of the rotating drum, and a closed-loop controller coupled to the motor and the encoder. The repetitive controller is coupled to the closed-loop controller in a secondary control loop, and is designed to compensate for the periodic disturbances in the rotation of the rotating drum. In another aspect of the present invention, a method of controlling the velocity of the rotating drum in an electrophotographic device to reduce banding artifacts includes using a primary controller in a primary control loop to control the rotational velocity of a rotating drum in the electrophotographic device, and using a repetitive controller in a secondary control loop to control the primary controller to reduce the effect of periodic drum rotational velocity fluctuations in producing banding artifacts.